Hepatocellular carcinoma (HCC) is a leading cause of cancer-related deaths. Although many cases of HCC are induced by chronic alcohol abuse or environmental toxins, approximately 80% are caused by infection with either hepatitis B or hepatitis C virus (HCV). The burden of HCV infection worldwide is very large, with most of the personal and economic burden yet to come, as cirrhosis and HCC take years to develop. Approximately 170 million people are infected worldwide with HCV. Most infected individuals do not clear the disease, but develop chronic infections that often lead to end-stage liver disease and HCC. Current treatment is limited to co-treatment with ribavirin and interferon [unreadable], a therapy that is expensive and ineffective in 50% of infected individuals. Therefore, there is an urgent need to identify new and accessible viral and cellular targets for therapies against HCV and HCC. It has been shown recently that cellular microRNAs, especially those in the liver, can be readily inactivated by direct intravenous delivery of modified oligonucleotides. Thus, our finding that liver-specific microRNA miR-122 interacts directly with the viral 5'noncoding region of HCV and that this interaction is essential to maintain intracellular abundance of the viral RNA, identifies a promising target for antiviral therapy. What are the host proteins required for microRNA expression in the liver? Recently, it has been found that host cell protein RCK/DDX6, a member of the DEAD box helicase family, is important for microRNA-mediated gene regulation in mammalian cells and that both miR-122 and RCK are greatly overexpressed in HCV-induced HCC in patient livers. Our preliminary results have shown that RCK is an essential positive regulator of HCV gene expression;depletion of RCK results in loss of HCV proteins. In this proposal, we propose to examine the mechanism by which RCK upregulates HCV gene expression at an apparently post-transcriptional step and by which RCK affects microRNA abundance and intracellular distribution in uninfected and HCV-infected liver cells. Using novel in situ hybridization technology, we will determine the effect of RCK expression on microRNA localization to specific cytoplasmic compartments known as P-bodies and stress granules. Finally, we propose experiments to identify targets for microRNAs whose abundance and distribution is affected by RCK, These studies will develop novel tools for HCV and HCC and are likely to point to new targets for antiviral therapy.